My PC won't turn on even after troubleshooting. Mobo or power supply issue

Hello! I have a common problem, but I can't seem to find the right answers! I am fairly experienced with PC hardware. I've been troubleshooting this problem for a couple of hours now; I checked if everything was installed properly on my motherboard and everything is plugged in quite snug! I'm torn between it being a power supply problem or a motherboard problem.

Let me tell you the series of events that happened within the last week;
About 5 days ago,I noticed that a TON of dust was caked on my heat sink fan and dust had formed a nice layer on my motherboard and graphics card. I gently cleaned it out with canned air, and cleaned out the dust from my heat sink with a q tip, pretty much just collecting dust slowly. My PC was abnormally cool as it was, so after the clean up it was doing great.
Last night I was up until about 4AM doing work and my PC was running perfectly. It was cool, it was running smooth. I left it on over night so I didn't have to exit the work I had left open; I knew I was getting up rather early in the morning to continue.
I went to go back to work at 11:30AM and my PC was off! I thought someone in my house had turned off my PC to conserve power; or it magically decided to turn off.
It decided to magically turn off, and break. Hah.
My motherboard and ethernet connection slots have little green lights on. Power is going through.
While troubleshooting, I came across this interesting occurrence. When I left my power supply off for more than 10 seconds, and turn it back on; my heat sink and cooling fans would temporarily spin for about a second. I can hear a very slight hum for a bit, and it would end. My PC won't turn on at all. Nothing hums when I press the power button, the screen doesn't flash, nothing budges.

I hope I was descriptive enough! If anyone could help a poor girl fix her PC that would be wonderful!